1. Field of the Invention
The present invention relates to a planetary ball mill and, more particularly, to a continuous air-swept type planetary ball mill in which feed to be ground is supplied and ground continuously.
2. Prior Art
In a generally known construction of a planetary ball mill, a plurality of mill pots respectively revolving following the rotation of a main shaft are disposed evenly around the main shaft (symmetrically in the case of two mill pots and radially with equal distance from the main shaft in the case of three or more), and each of these mill pots also rotates on its own axis. To be more specific, it is representative that a planetary gear is mounted on the periphery of a mill pot revolving together with the main shaft, and a sun gear mating with this planetary gear is separately rotated or fixed, thereby the pot is caused to revolve around the sun gear and rotate on its own axis simultaneously.
In general tumbling ball mills, balls serving as grinding media and feed to be ground undergo a cascade motion in a cylinder, and the feed is ground as a result of compressive collapse and frictional abrasion due to gravitational drop of the balls. On the other hand, in planetary type ball mills, the grinding speed is remarkably improved as compared with the tumbling ball mill by the synergistic effect between centrifugal force due to revolution and rotation, and Coriolis' force, and this results in the production of fine particles in a short time.
In particular, grinding force by high speed revolution and rotation is remarkable in planetary ball mills. For example, when operating for a few minutes after feeding silica of several millimeters in grain size, fine particles of several microns average diameter can be obtained.
As mentioned above, a planetary ball mill is characterized by the exhibition of strong grinding performance in a short time. A serious problem, however, exists in the conventional swept-type ball mill that utilizes air flow through the mill for continuously supplying a feed this problem being that, collection of the ground feed after grinding is difficult.
More specifically, in a generally known configuration of a conventional planetary type ball mill, a feed which has been already ground inside of a mill pot is guided from a discharge port of the mill pot to a discharge chute, and then is further carried over to a product discharge zone utilizing air flow. A sealing member is usually interposed between the revolving part including the mill pot rotating on its own axis, and the discharge chute so as to prevent an air leak which negatively affects the air-swept function. A problem, however, exists in that complete prevention of air inflow is very difficult because of the large diameter of the sealing member.
In the prior art as disclosed in the Japanese Patent Publication (examined) No. 34-7493, the feed which has been ground in the mill pot is transferred to a collector through an output tube, common basic portion and suction tube by utilizing air flow; then the ground product is collected after separating the air.
In the prior art of the above construction, since the output tube and common section revolve together with the main shaft while receiving the drive force from a motor, it is difficult to minimize a certain qunatity of air inflow from outside to inside at the connecting part between the revolving and rotating mill pot, and the output tube, as well as at the connecting part between the output tube and the stationary suction tube, in spite of the sealing. There will be another possibility that air is blown from the connecting part into the inside in the form of seal air. Accordingly, the quantity of air running through the suction tube is largely increased as compared with the quantity of air passing through the mill pot. As a result, the capacity of the collector and a rear end blower should be increased to meet such a situation.
Moreover, since the air quantity passing through the mill pot is largely variable depending upon the state of sealing, if inflow of air from the outside is large, the air quantity passing through the mill pot is reduced whereby the residence time of feed in the mill pot is prolonged, eventually resulting in over-grinding and agglomeration of the feed thereby negatively affecting the product quality. On the other hand, if the air quantity passing through the mill pot is excessively large, there is a possibility that a coarser feed is will be discharged as the final product without sufficient grinding. Anyway, there remains a further problem of mixing some product with the final product that is not conforming to a predetermined quality standard of the product.